Methods and usage of promise async and await

1.promise The process of changing a program from asynchronous execution to synchronous execution
For example, you can write a simple program to

<script type="text/javascript">
function one(){
return 'I am one'
}
function two(){
// Simulate a network request 
setTimeout(()=>{
return 'I am two'
},3000)
}
function three(){
return 'I am three'
}
function run(){
console.log(one())
console.log(two())
console.log(three())
}
// Call run function 
run()
</script>

here , The result of printing is determined by

I am one 
I am two
I am three

Turned into

I am one
undefined
I am three

Because two There is a network request , But at this time, I do not output the results

Because there's only one run Method , Not until 3 Seconds later, I'll execute it again

To solve this problem , You need to turn asynchronous execution into synchronous execution

 Need to change function two 
function two(){
// It's a standard promise Method ,resolve Is a request returned successfully ,reject Is what the request fails to return 
return new Promise((resolve,reject)=>{
// Here is also a setTimeout To simulate a network request 
setTimeout(()=>{
resolve ('I am two‘’)
},3000)
})
}

The state at this time is

Status at this time Because there is no statement , Must let two After performing To perform three

 At this point we are going to  run Change the status , Is to declare that there is promise Requested 
async function run(){
console.log(one())
console.log( await two())   // This means waiting two After execution , To perform three 
console.log(three())
}

His output is , display output

I am one

Wait for three seconds and output

I am two
I am three